The intervertebral disc (IVD) is the structure that connects contiguous vertebrae from C2 to the sacrum. It is composed of three regions: the outer annulus fibrosus, the inner nucleus pulposus, and the cartilaginous end plate.

In 1952, Hansen classified IVD disease into two types of disc herniation. Hansen type I IVD disease refers to herniation of the nucleus pulposus through the annular fibers and extrusion of nuclear material into the spinal canal. Type I disc disease is commonly associated with chondroid disc degeneration and is usually described in small, typically chondrodystrophoid breeds of dogs(Pekingese, Cocker Spaniel, Dachshund, Beagle...). However, some reports indicate that Hansen type I extrusions can also occur with some frequency in larger, non-chondrodystrophoid breeds. Type I extrusions occur acutely, causing dorsal, dorsolateral or circumferential compression of the spinal cord. In acute disc extrusions soft, but calcified disc material is found in the spinal canal, frequently associated to extradural hemorrhage.Hansen type II IVD disease refers to protrusions of the outer annulus fibrosus and is commonly associated with fibroid IVD degeneration. Type II disc disease is more common in old, non-chondrodystrophoid dogs, it may be asymptomatic or may present as a chronic condition with slowly progressive signs.

Degeneration can occur in all discs in the vertebral column. Chondroid metaplasia in chondrodystrophoid dogs can start as early as at 2 months of age, and be complete by 1 year. Fibroid metaplasia, on the other hand, is characterized by slow fibrous changes that begin near birth and progress for about 7 years. Both, fibroid and chondroid degenerations cause loss of water and elasticity of the disc. Genetics influences the risk of IVD herniation/protrusion. Other factors include spinal length, degree of muscular fitness and body weight.

Discs are usually herniated or extruded dorsally. The mass of nuclear material is forcibly ejected into the vertebral canal and causes concussion and compression of the spinal cord.

CERVICAL INTERVERTEBRAL DISC DISEASE

Clinical Signs

The incidence of type I IVD disease is reported to be between 13.9 and 25.4%, with C2-C3 being the most common disc involved. Acute cervical pain is the most common clinical sign. Low head and neck carriage, neck guarding, stilted and cautious gait, and cervical muscle spasms are the more common clinical manifestations. Even with large disc herniations, cervical pain may be the only clinical sign present. Another common clinical sign is radicular pain, observed with impigement of nerve roots C5-C8 by the extruded disc material. The animal´s gait may be ataxic, tetra- or hemiparetic, with normal or exagerated spinal reflexes in the pelvic limbs and normal/exagerated (C1-C5 herniations) or depressed (C6-T2 herniations) spinal reflexes in the thoracic limbs. Pain perception is usually preserved in all lims with cervical disc herniations. Cervical pain might be elicited by palpation of the cervical paraspinal muscles or by cervical manipulation.

In large dogs, cervical disc disease is usually caused by Hansen type II disc protrusions occurrying at disc spaces C5-C7. Clinical signs are then insidious and progress more slowly. Cervical pain is present but less severe than with acute herniations. Spinal reflexes in the thoracic limbs are commonly depressed, specially flexor reflexes.

THORACOLUMBAR INTERVERTEBRAL DISC DISEASE

Clinical Signs

Hansen type I IVD disease occurs more commonly at the thoracolumbar junction, with reported incidences between 66 and 83.6%. The more commonly affected disc spaces are T11/12-L1/2. Clinical signs are usually acute in presentation. Signs include thoracolumbar paraspinal pain, paraparesis (ambulatory or non-ambulatory) or paraplegia. Voluntary urination is frequently lost in paraplegic dogs. Pain perception in the pelvic limbs may or may not be present in paraplegic dogs. Spinal reflexes in the pelvic limbs are normal or exagerated with T3-L3 disc herniations and depressed with L4-S2 herniations. Hansen type II thoracolumbar disc disease causes the same neurologic signs, but these are chronic and slowly progressive.

Diagnosis

Diagnosis of IVD disease is obtained from signalment, history, and neurologic examination. Main differential diagnoses include trauma, vascular causes (fibrocartilaginous embolism), degenerative myelopathy (thoracolumbar IVDD), infectious/inflammatory conditions (discospondylitis, meningitis/meningomyelitis) and neoplasia.

A minimum data base (CBC, serum biochemistry, urianalysis, thoracic radiographs, abdominal ultrasound) should be the first diagnostic step.

Survey spinal radiographs can provide helpful information regarding the site of disc herniation. However, myelography remains as the main diagnostic tool to diagnose IVD disease. Myelography can accurately determine the site and lateralization of disc herniation/protrusion. In acute herniations, swelling of the spinal cord at the site of herniation may make accurate localization difficult. In these cases, CT-myelography can be very helpful. MR imaging alone or as an adjunct to myelography can also be used to more completely delineate the presence of extruded disc material in the spinal canal.

Treatment

Intervertebral disc disease is a surgical disease. Surgical removal of the extruded or protuded disc material is the treatment of choice for overall management of intervertebral disc disease. Ventral slot decompression os the most commonly performed and preferred technique for removal of extruded cervical disc material. Dorsal or dorsolateral procedures provide more spinal cord decompression than a ventral slot and easier access to laterally extruded disc material. However, removal of ventrally extruded disc material is difficult without excessive spinal cord manipulation. Urgical procedures to decompress the thoracolumbar spinal cord and remove the herniated disc material include hemilaminectomy, pediculectomy, and dorsal laminectomy. As with cervical disc disease, removal of the herniated disc material is difficult through a dorsal laminectomy without excessive manipulation of the spinal cord, unless the herniated disc is located dorsally or very lateralized. Fenestrations of all the cervical discs and, at least 2 discs cranial and 2 discs caudal to the thoracolumbar disc herniation are recommended to prevent further disc herniations in chondrodystrophoid dogs.

REFERENCES

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